Films comprised of polymers of reactive acrylate and methacrylate monomers are well known and possess many diverse uses. Such films may for example, be utilized by the coating industry for protective and/or decorative coatings on a variety of substrates such as, for example, paper, metal and wood.
Coatings of acrylate and methacrylate polymers, until recently, were prepared by coating a substrate with a solution of the polymer composition in a suitable organic solvent and baking in a furnace to drive the solvent from the resulting film. Due to the stringent requirements concerning water and air pollution and a growing concern to conserve energy, the coating industry has turned to newer methods of forming such films.
Today, coatings of acrylate and methacrylate polymers are often prepared by radiation curing of a monomer or oligomer composition utilizing ultraviolet light. Since radiation curing can be carried out in the absence of conventionally used solvents and does not require a post heating step, it is attractive from the viewpoint of environment protection and energy conservation. Radiation curing of acrylate and methacrylate films has been the subject of considerable research and many scientific articles; for example, Rybny et al, Journal of Paint Technology, Vol. 46, No. 596, pp 60-69 (1974).
Radiation curing of monomers, such as acrylates and methacrylates, although attractive from the viewpoint of environmental safety and economy, is not without disadvantages. For example, in radiation curing, the degree of polymerization is directly related to the depth to which the light penetrates the monomer composition. Therefore, complete polymerization is only possible when utilizing relatively thin coatings of monomer composition on the substrate. Thicker films will polymerize to a considerably lesser degree at the base of the films resulting in a loss of adhesion to the substrate.
Another related disadvantage of radiation curing is that the monomer composition must be relatively free of substances which would make it opaque. Therefore, certain substances, otherwise desirable for monomer compositions for a particular utility, cannot be utilized with radiation curing as their presence would retard light penetration, resulting in incomplete polymerization.
As an alternative to radiation curing, monomer compositions, such as acrylates and methacrylates, may be cured by exposure to electron beam.
Electron beam curing, regardless of the type of particle accelerator source to be utilized, requires a very substantial investment in apparatus. Although electron beam curing is effective with coatings which are non-conductive or which contain pigments, its effectiveness is due to the very large voltage of the beam itself. The large amount of energy required to generate a beam of such voltage is a significant disadvantage in today's economic environment. These disadvantages can be particularly significant for certain conventional applications of such films; e.g., the production of wood siding or coating of metal wire.
This invention provides a method of curing acrylate monomer compositions which is not characterized by any of the above-mentioned disadvantages. Such compositions are cured in accordance with this invention by minimal exposure to gaseous sulfur dioxide.